Simplex data communication systems, also known as one-way or passive systems, are employed in connection with paging. Generally speaking, one or more transmitters broadcast data communications, referred to herein as "messages", which originate from one or more communication units. The originating communication units are referred to herein as "source units". The messages include data which identify specific receivers (e.g., pagers). A population of receivers receives the broadcast messages. When one of the population of receivers identifies a message directed to it, it informs the person or device possessing the receiver that an incoming message has been received. The receiver often displays a numeric or alphanumeric representation of information contained within the message. The specific receiver to which a message is directed is referred to herein as a "target unit". The person or device possessing a transmitter or receiver is referred to herein as a "subscriber". Due to the simplex nature of such systems, the system does not know when a target unit fails to receive a message directed to it.
On the other hand, these simplex systems have many desirable features. Due to the absence of a transmitter and signal transmission capabilities in the receiver, a small, light-weight, readily-portable, and inexpensive unit results. However, one problem with a portable unit is that the unit's power is limited as the power is typically provided by batteries.
Conventional paging systems experience a problem related to a limited range. A paging system works optimally when its pagers reside within the area covered by the system's transmitters. When subscribers travel outside this area, their pagers cannot usually receive calls. This problem is exacerbated by the simplex nature of the system which prevents the system or the source unit from knowing whether the call was actually received by the target unit.
A related problem is that of limited paging capacity. As an area of coverage increases to better serve subscriber needs, the population of pagers likewise increases. As the population of pagers increases, the number of data communications increases. Thus, as the coverage area increases, a point of diminishing returns is reached. The number of data communications may become so great that an unacceptable delay in the delivery of messages is experienced.
Another problem with conventional paging systems is concerned with multipathing, interference, signal reflections, and the like. Generally, pagers are worn by subscribers while engaging in their normal daily activities. These normal activities may place the subscribers inside automobiles or buildings, near large electrically-grounded structures, and in other spaces that electromagnetic signals have trouble penetrating. Consequently, the pagers' abilities to receive messages varies considerably. To maximize a pager's ability to receive messages in all surroundings within a coverage area, paging systems are typically designed to utilize a low data-rate frequency shift key (FSK) modulation scheme in which transmitter power is boosted to a level that achieves an acceptable link margin. In addition, in simulcast paging systems the number of system transmitters may be multiplied and geographically distributed throughout the covered area. The determination of an adequate number of transmitters and adjustment of power levels to achieve an acceptable link margin throughout the coverage area is typically a slow, trial-and-error process. This process is slowed even further by the simplex nature of the system which prevents system operators and the call senders from knowing whether messages are actually being received by the target units.
The use of satellites in paging has also been proposed. Satellites would contain system transmitters and would broadcast messages to populations of target units in a manner which in some respects is similar to that used by fixed, ground-based transmitters. The use of satellites may address the limited range problem of conventional pager systems. Because a satellite may be located far above the earth's surface, the satellite's RF transmissions may more easily reach target units without interference. For example, geostationary satellites or satellites that move with respect to the surface of the earth (e.g., low-earth or medium-earth orbiting satellites) may theoretically be used. However, the use of satellites presents its own problems.
Satellites are typically constrained to using only low-power transmissions. Geostationary satellites are currently used to broadcast to terrestrial repeaters which can then transmit high power signals to nearby pagers. Proposals have been made to integrate a satellite with terrestrial transmitters in a simulcast paging system. However, this requires a satellite in a geostationary orbit, where the satellite is positioned a vast distance above the earth, where the satellites' transmission signal is extremely weak at the surface of the earth, and where the area of coverage on the earth's surface is exceptionally large.
Satellites orbiting nearer the earth could also contain system transmitters. Because the coverage of a satellite decreases as it nears the earth's surface, multiple satellites would be required to provide an appreciable area of coverage on the surface of the earth. A system having multiple satellites presents the problem of coordinating messages and routing them to a proper satellite that is servicing a target unit.
What is needed is a simplex data communication system capable of coordinating and delivering messages without interference to target units located in geographically-diverse areas outside the range of a single system transmitter. What is further needed is a system with the ability to deliver messages to a large area of the earth's surface without invasive antenna structures. What is further needed is a system that provides a high probability that a target unit will receive messages addressed to it.